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Have my car running with ZERO KNOCK!!!

Disclaimer: This has not been thoroughly tested, and is not guaranteed safe. Take this info with a grain of salt. If you blow your car up, it's not my fault or my problem.

So awhile back i started logging the knock readings with the TQ App. On 87 Octane the car was constantly experiencing 2-3* of KR in literally every loaded situation (even light load), and running very low total timing. On 93, the car switches to the high octane timing maps, and even with the added timing KR was improved, but still had frequent, repeatable, consistent knock.

There is only one E85 station within 50 miles of me, and their prices are high because they are the only one around. It's very inconvenient for me to go there and is completely out of the way, so price aside i haven't had interest in it despite the amazing performance (proven equal to leaded 112 octane in high boost applications). Yesterday i started thinking about blending a low dose of E85 with 93 and running it on the stock tune. Winter blend is closer to E70, but we will disregard that for the purpose of this post.

Now, i'm going to get a bit technical here so bear with me.

The first thing you need to understand is that the ECU adjusts the fueling automatically based on the o2 sensors. If the fuel mix ethanol content increases, the car will run lean for a bit. The ECU will immediately compensate by adjusting short term fuel trims, or STFTs for short. After doing this for awhile, the ECU will then begin to adjust the long term fuel trims (LTFTs) in an attempt to regain homeostasis if you will, and zero out the STFTs.

Now, at full throttle the O2s aren't used, but the fuel trims are, which means while the O2 sensors are not providing information used in fueling, the information previously provided is used in the calculation.

What this means is, the car will automatically retune its fueling at both part and full throttle, for the changes in ethanol content...within reason.

If you just fill the tank with E85, the car will be well beyond the ECUs acceptable fuel trim parameters. I'm not sure what those parameters are, or what point will trigger the CEL.

I did some basic math at the gas station on my phone, and decided 10% E85 blended with 90% 93 Octane would be well within the ECUs limits, while still increasing the knock threshold by a good amount.

Info:

On the 15.6 Gallon tank, 1.5 gallons of a typical 93 octane fillup are ethanol. 1.5 Gallons of E85 is 1.275 Gal of ethanol, and .225 gal of gasoline...so adding 1.5 gallons of E85 makes it 2.675 Ethanol, to 12.8 Gallons of 93.

This comes out to E21, for a total of 21% ethanol.

E85 is 105 octane, but runs like 112 in a boosted engine thanks to its cooling properties, so 112 is the number to use.

So with 10% E85, the effective octane is just under 95 octane for the E21.

Now, E85 requires roughly 30% more fuel in most applications, but compared to the E10 we're all already burning, we're only adding 11% E85, which according to my road side cell phone math, would require 3.3% additional fuel, netting a 1.9% gain in "real" octane.

3.3% should be well within the safe margin the ECU can compensate for before it notices anything fishy. Once the LTFTs are adjusted +3.3%, the fueling at WOT will be adjusted accordingly.

Now, because E85 has less energy than gasoline, and i can't adjust the timing map to regain the lost power, i want to use as little E85 as possible to prevent knock, and no more than needed. This will allow the car to make more power, safer. Overkilling the E85 without advancing timing will cause the car to lose power, and run inefficiently.

So far i've driven about 40 miles. The first 10-15 were gentle to let the car start to adjust. Then i beat the **** out of it.

I've done dozens of full throttle pulls at any and every rpm.

Results: No CEL, and i can't make the car knock. I've tried like ****. I slowed down to 25mph, floored it in 6th gear, and didn't lift until i was well above the speed limit on the highway...not a single fleck of knock while generating as much load as possible. 6th gear brake boosting at WOT, 0 knock.

Car is on the high octane map (obviously) and I couldn't make the car knock if i had to. With 93 alone and the abuse i just gave it, i would have had consistent 1.5-2.5* of knock retard.

Next time i'll try a little less E85 and see how it runs. Like i said the goal is as little E85 as possible.

Time for Xmas dinner, best to all of you and your families this holiday season.

It makes no sense that i'd be running rich. Even so, WOT AFR is extremely lean (13.7-12.0 for most of the RPM range, gets down to 11:1 above 5500-6000) so more fuel would be very welcome.

My LTFTs on 10% E85 range from 3-5.5% depending on RPM, so in theory, AFR should remain exactly the same. Without a wideband i can't guarantee that they are, but they should be according to the math and the observed LTFTs

I'd been experimenting with 87 in the cold weather, and my car was running TERRIBLE in traffic on the highway - pulsing and surging every time I slowed down. I mixed with a tank of 93 because it was pissing me off, and voila! Instantly better.

There's 100 octane racing fuel at a station near me (@ $6/gallon). I'd like to run my car down to empty and try 2-3 gallons of it just to see what it can do with the Torque app.

Could you notice any difference in performance without knock with the E85 mixture?

There's no way i'd have negatives when running E20....so therefore LTFTs are applied at WOT.

It makes no sense that i'd be running rich. Even so, WOT AFR is extremely lean (13.7-12.0 for most of the RPM range, gets down to 11:1 above 5500-6000) so more fuel would be very welcome.

My LTFTs on 10% E85 range from 3-5.5% depending on RPM, so in theory, AFR should remain exactly the same. Without a wideband i can't guarantee that they are, but they should be according to the math and the observed LTFTs

How does it make no sense you'd be running rich? Your positive LTFT's are applied to your open loop PE fueling, while negative LTFT's never will. Therefore, your PE fueling is going to be richer than commanded based on this alone. This is a safety measure to make sure you don't blow up your engine.

How does it make no sense you'd be running rich? Your positive LTFT's are applied to your open loop PE fueling, while negative LTFT's never will. Therefore, your PE fueling is going to be richer than commanded based on this alone. This is a safety measure to make sure you don't blow up your engine.

Think this through with me, because i'm not quite following you.

If the stock maps are perfect in closed loop, the FTs will be 0, meaning the engine will get 100% of the commanded fuel. By increasing the ethanol content to E21, i'm increasing the required fuel 3% to run at stoich/ 1.00 lambda.

The ECU sees that its maps are consistently lean by 3%, and adjusts the LTFT to +3%.

Now commanded lambda x 1.03 = stoich.

Open loop map with E10 and 0 fuel trims means the real AFR should be extremely close to the ECUs commanded AFR/1.00 lambda.

The same maps on E21 would be lean with 0% LTFT.

Given that closed loop running had adjusted the LTFT to 3%, what i now have (in basic terms) is

E21 (Command AFR X 1.03)=Real AFR

vs

E10 (Commanded AFR X 1.00)=Real AFR

The ECU is injecting 3% more fuel at WOT, but the engine requires 3% more fuel to have the same AFR because if the ethanol change, so the engine should be running with the same AFR despite the increased fuel injected.

That was slightly more advanced than I am with my knowledge, but from what I understand this is a pretty cool experiment/finding.

Its posts like this that make me love forums like CruzeTalk.

Thanks man. It's posts like this that make me continue posting the things i experiment with.

Originally Posted by jblackburn

Hmm...quite interesting.

I'd been experimenting with 87 in the cold weather, and my car was running TERRIBLE in traffic on the highway - pulsing and surging every time I slowed down. I mixed with a tank of 93 because it was pissing me off, and voila! Instantly better.

There's 100 octane racing fuel at a station near me (@ $6/gallon). I'd like to run my car down to empty and try 2-3 gallons of it just to see what it can do with the Torque app.

Could you notice any difference in performance without knock with the E85 mixture?

It's tough to say. With the cold temps we've been having lately i have been seeing less knock on 93, but it's still a very frequent thing in high load situations.

With the E85 mix the engine is definitely the smoothest it's ever been, but i don't think it's making any more power than 93. I actually think it might be making the same or slightly less thanks to the lower energy content of the E85 and still on the stock timing map.

Any more E85 than needed to prevent knock will start to swing performance the other way. This might be a great blend in hot summer temps to get the most bang out of it, but right now i think i went a little to far with the E85.

The good news is i have about 100 miles on it so far and the ECU seems to be compensating just fine with no codes/issues.

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